The objective of this proposal is to determine the developmental changes in ionic exchange in the neonatal mammalian myocardium and the effect of hypoxia on ionic exchange. Experiments will be performed in the arterially perfused intraventricular septum of the neonatal rabbit and dog. Variables describing mechanical function will be recorded on a beat-by-beat simultaneously with changes in ionic uptake by the tissue and ionic efflux. The uptake of gamma emitter ions (58CO, 47Ca, 42K, 133Ba) will be monitored in the whole septum by gamma counting system. Changes in ionic efflux will be determined from effluent analysis. This study will delineate the developmental changes in sarcolemmal permeability to ionic exchange. It should also delineate the effect of hypoxia followed by reoxygenation on the function of the sarcolemma, sarcoplasmic reticulum and mitochondria.